This invention relates to novel methods of producing flat and curved optical elements, in particular elements of extremely high precision, using multilayer thin films for use with soft and hard x-rays, cold and thermal neutrons, and the optical elements achieved by these methods.
Thin film technology has been widely used to control the reflection and transmission of visible light. However, in the wavelength range of x-rays and neutrons the use of thin films has only recently become practicable. X-ray and neutron optics has presented many challenges to scientists including the inability to reflect at near-normal incidence, poor quality of paraboloid or hyperboloid optical elements used for grazing reflection and lack of sources. Recent advances in the quality control of Layered Synthetic Microstructures (LSM), or multilayers, allows the use of these structures as x-ray and neutron mirrors.
X-ray diffraction from multilayer mirrors is analogous to x-ray diffraction from perfect crystals where the lattice planes are located in the nodes of the standing wave produced by the superposition of incident and reflected (diffracted) waves for enhanced diffraction efficiency. Multilayer mirrors can be considered as an extension of natural crystals for larger lattice spacings. Therefore, as for crystals, x-ray photons will be reflected from multilayer structures only if the Bragg equation is met: EQU n.lambda.=2d sin (.theta.)
where
.lambda.=wavelength of the incident radiation PA1 d=layer-set spacing of a Bragg structure, or the lattice spacing of a crystal PA1 .theta.=angle of incidence PA1 n=the order of the reflection
The structure of a crystalline solid, a regular three dimensional array of atoms, forms a natural diffraction grating for x-rays. The quantity d in the Bragg equation is the perpendicular distance between the planes of atoms in the crystal. The construction of an artificial diffraction grating with a spacing on the order of the x-ray wavelength was impossible at the time W. L. Bragg derived his foundational equation. However, crystalline structure can now be imitated by thin film multilayers, so x-ray diffraction is no longer limited to structures with naturally occurring d spacings.
In order for a multilayer structure to reflect by imitating a crystal structure, a light element of the lowest possible electron density is layered with a heavy element of the highest possible electron density. The heavy element layer acts like the planes of atoms in a crystal, as a scatterer, while the light element layer behaves like the spacers between the planes of atoms. A further requirement of these two elements is that they do not interdiffuse.
Multilayers possess advantages over natural crystalline structures because by choosing the d spacing of a multilayer structure, devices may be fabricated for use with any wavelength and incidence angle. Crystals also possess poor mechanical qualities such as resistance to scratching.
X-ray optics has benefitted greatly from three variations on a multilayered optical element: multilayers on figured or curved optical elements, depth graded multilayers and laterally graded multilayers.
By varying the d spacing laterally across the surface of a figured optic, x-rays of the same wavelength can be reflected from every point on the surface, even where the angle of incidence changes across the surface. At each point, the angle of incidence and the d spacing is manipulated according to the Bragg equation. Depth grading is used as a means for broadening of the band pass, therefore increasing the integrated reflectivity of a particular multilayer structure.
Two sources of error will profoundly affect the performance of an x-ray optical element. First, the curvature of the element is difficult to produce exactly and will be subject to a tolerance range. Second, although great improvements have recently been made in techniques for quality control of evaporated and sputtered films, imperfections in d spacing will always exist.
Errors in the surface curvature of the element will partly destroy the image. The reflectivity of the element will also decrease because the angle of incidence will be different than that calculated. The d spacing error will also result in decreased reflectivity.
Accuracy to a fairly low tolerance is required from both the d and .theta. input in the Bragg equation. However, errors in d spacing are difficult to distinguish from errors in curvature in the final products. The result is that numerous elements will probably be discarded before an acceptable optical element is produced. Without the ability to determine whether the error lies in the shaping or deposition processes, the production process cannot be corrected.
The current invention is comprised of a figured optical element and unique methods used to produce a multilayer structure on this element. The optical element consists of a curved substrate upon which a plurality of layer sets are produced. In the production of this element, the causes of imperfections can be isolated and the d spacing and/or angle of incidence can be adjusted to compensate. The result is unprecedented performance of an x-ray optical element.
One method involves characterizing the surface of the optical element before multilayers are deposited onto it. Calculations are then performed so that the layer d spacing will compensate for errors in the curvature. As a result, the reflectivity of the surface will be preserved.
In an alternate method, a flat optical element will be coated with thin multilayers whose spacing has been calculated to achieve the desired effect on a beam of x-rays for an element with a known curvature. Then the deviation of the actual d spacing from the calculated multilayers will be found. Using this information, an adjusted curvature for an element can be calculated to compensate for the error in d spacing.
An additional advantage of this invention is its application to x-rays in the soft x-ray (about 10 to 200 angstroms) and the hard x-ray range (about one one-thousandth of an angstrom to 10 angstroms). Previously disclosed elements have been limited to use with soft x-rays and extreme ultraviolet rays. The appreciably shorter wavelength of hard x-rays demands previously unattainable accuracy in optical elements.
Proposed applications of such optical elements include spectroscopy and diffractometry, in particular, a diffractometer using a parabolic multilayer mirror with lateral grading d spacing which reflects a parallel beam of defined wavelength. Optical elements would also be applied to focusing optics, for x-ray lithography and microscopy, in particular, optics for high resolution scanning x-ray microscopy, point to point imaging optics including multi-element systems, an optic for monochromatization of broad-band radiation, synchrotron radiation in particular. Many medical applications are also contemplated, in particular, as power filters to eliminate undesired energy or use in radiography where a high contrast image is desired.
These optical elements can also be used for transformation beams of cold and thermal neutrons. In particular, they can be used for increasing density and uniformity of neutron flux or separation of the neutrons with different spin.
Additional objects and advantages of the invention will become apparent from the following description and the appended claims when considered in conjunction with the accompanying drawings.